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Question DetailsAsked on 2/18/2018

So I moved in to a condo a few months ago. One of the things that I liked was that it had a brand new furnace and AC. But after a few weeks, I decided to peek in to the ducts to contemplate whether an air-duct cleaning was in order (whether through what I could do or get a professional company).

Anyway, I noticed that all the ducts have some kind of insulation on the INTERIOR of them. First it looks a bit yellowish then the most central part is some kind of black/dark furry insulation. It looks to be fiberglass.

Should I save my money and plan on replacing the duct-work? It's a small 1-bedroom condo so shouldn't be that expensive and seems to be better than having ducts cleaned and risking all that insulation to get lose (AFTER the company leaves lol).

1 Answer

I've been wondering when a question would show up here about interior-insulated ducts - probably one of the silliest ideas to ever come along, especailly when doen after-the-fact because the insulation dramatically reduces the airflow capacity/resistance of ducts which were presumably right-sized in their original configuration.

You might contact the furnace installer (commonly a sticker or magnetic business card on the furnace housing) to see if they knew about the insulation and incorporated its existence into their furnace sizing/blower power calculations, or if may they put it in.

The dark part would likely be dust buildup - if much is yellow I would guess it was installed with the new furnace, because usually the interior of a duct will be pretty much flat gray (or rarely other colors in very high colored dust areas like in a painted desert, or in houses with badly shedding bright colored carpet), so I would guess it is fairly new.

Having the insulation in there, other than that it can absorb moisture during A/C operation (especially if installed with the fiberglass exposed to the airflow), and creates a fair amount more airflow resistance, does not in itself constitute a reason to remove it or replace the ducts. If your unit is working adequately and not running excessively (not more than about 30-40% of time in normal heating conditions, not more thn 80-90% of time in near record-low temps) I would not worry about it - and if the insulation is yellow then obviously you are not getting a lot of dust coming through so I see no reason to clean the ducts or replace them.

The insulation shouold not get loose - is usually installed as a bonded blanket mechanically fastened or glued to the inside of the duct, so should not come free. If you are concerned, there are replaceable filters which can be placed under the supply vent grills to trap dust coming from/through the ducts - just be sure they are free-flowing and do not cause a change of pitch of the blower on the furnace (which would indicate excessive back-pressure).

Just putting a filter in one or two ducts would show how much dust is getting in through the ducts - you can used just a sheet of quilting batting or roll medical cotton temporarily as a dust filter to see how much is getting in - just remove vent grill, stretch filter across (large anough that the grill edges clamp down on it), put grill back on. Make sure this is a supply vent - air coming out of it when furnace is on. Doing that on a return grill would just show how much dust you have floating around in the household air, from all sources combined.

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